In the description that follows references are made to certain compounds, devices and methods. These references should not necessarily be construed as an admission that such compounds, devices and methods qualify as prior art under the applicable statutory provisions.
The verification of the existence of a third form of carbon termed "fullerenes" in 1990 touched off an intense wave of research and development aimed at maximizing the potential of this "new" material. The term "fullerene" is often used to designate a family of carbon molecules which have a cage-like hollow lattice structure. These "cages" may be different forms, such as spheres ("buckyballs"), or tubes ("nanotubes"). See, for example, Robert F. Curl and Richard E. Smalley, Fullerenes, Scientific American, October 1991.
With the increasing importance of batteries for a wide variety of uses, ranging from portable electronics to power supply devices for spacecraft, there is a long-felt need for new materials with higher energy densities. The energy density of a material can be quantified by measuring the amount of electron-donating atoms that can reversibly react with the material. One way of obtaining such a measurement is by setting up an electrochemical cell. The cell comprises a container housing an electrolyte, one electrode made of the electron-donating material (e.g.--an alkali metal), another electrode made of the material whose capacity is being measured (e.g.--a carbon based material), and an electrical circuit connected to the electrodes. Atoms of the electron-donating material undergo an oxidation reaction to form ions of the donating material, and free electrons. These ions are absorbed by the opposite electrode, and the free electrons travel through the electrical circuit. Since the number of electrons "given away" by each atom of the electron-donating material is known, by measuring the number of electrons transferred through the electrical circuit, the number of ions transferred to the material being investigated can be determined. This quantity is the specific capacity of the material, and can be expressed as milliampere-hours per gram of the material. For example, the maximum specific (reversible) capacity of graphite to accept lithium is reported to be approximately 372 mAh/g. Because one lithium ion is transferred to the graphite electrode for every electron released, the specific capacity can be expressed in terms of the stoichiometry of the electrode material. For graphite, the electrode material can be characterized as LiC.sub.6. See, for example, J. R. Dahn et al., Mechanisms for Lithium Insertion in Carbonaceous Materials, Science, volume 270, Oct. 27, 1995.
Lithium intercalated graphite and other carbonaceous materials are commercially used as electrodes for advanced Li-ion batteries. See, for example, M. S. Whittingham, editor, Recent Advances in rechargeable Li Batteries, Solid State Ionics, volumes 3 and 4, number 69, 1994; and D. W. Murphy et al., editors, Materials for Advanced Batteries, Plenum Press, New York, 1980. The energy capacities of these conventional battery materials is partially limited by the LiC.sub.6 saturation Li concentration in graphite (equivalent to 372 mAh/g).
Carbon nanotubes have attracted attention as potential electrode materials. Carbon nanotubes often exist as closed concentric multi-layered shells or multi-walled nanotubes (MWNT). Nanotubes can also be formed as a single-walled nanotubes (SWNT). The SWNT form bundles, these bundles having a closely packed 2-D triangular lattice structure.
Both MWNT and SWNT have been produced and the specific capacity of these materials has been evaluated by vapor-transport reactions. See, for example, O. Zhou et al., Defects in Carbon Nanotubes, Science: 263, pgs. 1744-47, 1994; R. S. Lee et al., Conductivity Enhancement in Single-Walled Nanotube Bundles Doped with K and Br, Nature: 388, pgs. 257-59, 1997; A. M. Rao et al., Raman Scattering Study of Charge Transfer in Doped Carbon Nanotube Bundles, Nature: 388, 257-59, 1997; and C. Bower et al., Synthesis and Structure of Pristine and Cesium Intercalated Single-Walled Carbon Nanotubes, Applied Physics: A67, pgs. 47-52, spring 1998. The highest alkali metal saturation values for these nanotube materials was reported to be MC.sub.8 (M=K, Rb, Cs). These values do not represent a significant advance over existing commercially popular materials, such as graphite.
Therefore there exists a long-felt, but so far unfulfilled need, for a material having improved properties. There exists a need for a material having improved properties that make it useful in batteries and other high energy applications. In particular, there is a need for a material having a higher energy density than those materials currently being used in such applications.